This invention relates in general to a solenoid coil assembly included in a vehicle anti-lock brake system and more particularly to a solenoid coil assembly that provides compensation for manufacturing tolerances.
An anti-lock brake system (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of normally open and normally closed solenoid valves which are mounted within a control valve body and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. The pump is typically included within the control valve body while the pump motor is mounted upon the exterior of the control valve body.
An ABS further includes an Electronic Control Unit (ECU) which has a microprocessor. The control unit is electrically coupled to the pump motor, a plurality of solenoid coils associated with the solenoid valves and wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. The control unit is typically mounted upon the control valve body to form a compact unit which is often referred to as an ABS electro-hydraulic control unit or Hydraulic Control Unit (HCU).
During vehicle operation, the microprocessor in the ABS ECU continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the wheel speed signals for a potential wheel lock-up condition. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to actuate the pump motor and selectively operate the solenoid valves in the control unit to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
As described above, an ABS typically includes a plurality of solenoid valves for controlling the flow of hydraulic fluid to the vehicle wheel brakes. Solenoid valves are electrically actuated by supplying an energizing current to a solenoid coil assembly. A typical coil assembly includes a coil in the form of an insulated magnet wire wound on an insulated bobbin. The bobbin supports a pair of terminal leads. The ends of the coil magnet wire are wound upon the terminal leads. The terminal leads are connected through an electronic switch to a voltage supply. When the electronic switch is in a conducting state, current passes through the magnet wire and produces a magnetic field.
Solenoid valves also include an axially shiftable armature that is disposed within a valve sleeve. The solenoid coil assembly is carried by the valve sleeve. The armature is biased by spring to maintain a valve ball in a normally opened or closed position. The valve ball is adapted to cooperate with a valve seat member, which is provided in a valve body. The solenoid coil assemblies are typically enclosed within a cup-shaped a flux return casing. An annular flux ring is often disposed within an open end of the flux casing. The annular flux ring completes a magnetic flux path that is adapted to pass through the armature and the valve seat member.
To actuate the valve, electric current is supplied through the terminal leads to the solenoid coil. The current establishes a magnetic field in the armature, which pulls the armature against the force of the spring to open or close the valve ball. An interruption in the current causes the magnetic field to collapse. This allows the spring to return the armature to its normal position.
To insure proper operation of the valve, the armature and sleeve must fit within a close tolerance of the bobbin. The bobbin must fit within a close tolerance of the flux return casing. Moreover, the annular flux ring, the flux return casing, the armature, and the valve seat member must make sufficient contact with one another to assure an optimal flux path.
A plurality of valves are usually mounted upon a hydraulic control unit. Each of the valves is controlled by a separate solenoid coil assembly. The coil assemblies are typically controlled by an electronic control unit. The electronic control unit is often coupled to the coil assemblies via a lead frame or printed circuit board that supports a plurality of coil assemblies. The lead frame or printed circuit board includes a pair of holes for receiving the terminals of each of the solenoid coil assemblies.
A problem exists with positioning the coil assemblies relative to respective valves due to manufacturing tolerances. For example, the terminals of a plurality of coil assemblies are connected to a lead frame or a multi-chip module. A plurality of valves are mounted upon by a hydraulic control unit. Each of the coil assemblies, though connected to the lead frame or multi-chip module, must align with a corresponding valve sleeve. This often requires that a certain amount of play or spacing exist between the coil assembly bobbins and the valve sleeves as a result of manufacturing tolerances. The play reduces the magnetic field established in the armature. In addition, an inability to control the position of the flux return casings relative to their respective valve seats may result in insufficient contact between the flux return casings and the valve seats. This further reduces the magnetic field established in the armature.
A coil assembly is needed that fits snuggly about the valve cartridge armature and that encounters minimal axial translation resulting from manufacturing tolerances to maximize the magnetic flux through the valve armature.